At present, for example, solar cells or optical sensors generally employ photoelectric conversion elements that convert incident light to an electric signal by photoelectric conversion. The photoelectric conversion element employs a semiconductor, and when electromagnetic waves (light) whose energy exceeds the band gap of the semiconductor are incident to the photoelectric conversion element, electrons are excited in the semiconductor from a valence band to a conduction band, so that photoelectric conversion occurs.
For example, it is generally known that a-Si which is an amorphous semiconductor has absorption at approximately 700 nm in wavelength (absorption edge of light is at approximately 700 nm). That is, electromagnetic waves (light) whose wavelength is shorter than the absorption edge are absorbed by a photovoltaic material, so that photoelectric conversion occurs in the photovoltaic material. However, an actual device exhibits absorption up to approximately 820 nm due to improvement in processing methods and manufacturing methods. Accordingly, generation of a photovoltaic power can be expected also in a band approximately between 700 nm and 820 nm in wavelength.
FIG. 32 is a view showing measured values of an absorption ratio of a-Si (of 330 nm in thickness) with respect to the wavelength of light.
As shown in FIG. 32, a-Si exhibits peaks of absorption at approximately 520 nm or less in wavelength, and as the wavelength becomes larger from approximately 520 nm to 820 nm at the absorption edge, a-Si exhibits a smaller absorption ratio. This is because the interaction between light and electrons is weak at a region between the absorption edge and the absorption peak of the semiconductor, and so electromagnetic waves (light) at the region are more likely to be transmitted by a-Si. Consequently, a photoelectric conversion ratio drops between the absorption edge and the absorption peak of the semiconductor. Therefore, in order that the semiconductor sufficiently absorbs light between the absorption edge and the absorption peak, it is necessary to make the semiconductor thicker.
Recently, in order to enhance a light absorption ratio, there have been developed photoelectric conversion elements using photonic crystals as disclosed in Patent Literatures 1-4 below.
FIG. 33 is a view schematically showing a configuration of a solar cell disclosed in Patent Literature 1.
Photonic crystals are periodic structures with different dielectric constants which are artificially formed in a dielectric material with periodicity substantially equal to the wavelength of light.
As shown in FIG. 33, a solar cell 101 is designed such that photovoltaic material 103 is laminated on a distributed Bragg reflector (DBR) 102 and a photonic crystal structure 104 having a plurality of air holes is formed in the photovoltaic material 103.
A part of incident light i having entered the photovoltaic material 103 is regularly reflected by the photonic crystal structure 104 to be reflective light r0, another part of the incident light i is diffracted by the photonic crystal structure 104 to be diffractive light r1, and still another part of the incident light i is refracted by the photonic crystal structure 104 to be refractive light t.
Since the diffractive light r1 results from diffraction with an angle θ′ larger than the incident angle θ, the diffractive light r1 contributes to lengthening a light path inside the photovoltaic material 103. Furthermore, total internal reflection occurs at the interface between the photovoltaic material 103 and the outside air, so that the diffractive light r1 is resonated inside the photovoltaic material 103. Consequently, the photovoltaic material 103 exhibits an improved light absorption ratio.
Furthermore, the refractive light t and light reflected by the distributed Bragg reflector 102 to return to the photonic crystal structure 104 are reflected and go back and forth to cause resonance inside the photonic crystal structure 104 and absorbed therein gradually. This also improves the light absorption ratio.
As described above, in the solar cell 101, the incident light is resonated inside the photovoltaic material 103 and the photonic crystal structure 104 so that the light is absorbed, thereby improving the absorption ratio of the photovoltaic cell. In particular, by providing a resonance wavelength at a long wavelength side where the absorption ratio of incident light is small, it is possible to realize an absorbing body capable of absorbing sunlight with a wide wavelength range.
Furthermore, Non-patent Literature 2 below discloses a solar cell using photonic crystals with a band edge. The following explains a configuration of a solar cell disclosed in Non-patent Literature 2 with reference to FIG. 34.
A solar cell 200 shown in FIG. 34 is designed such that a photovoltaic layer 203 made of an organic material is provided with a photonic crystal and a band edge thereof is used so as to enhance absorption of the photovoltaic layer 203. As a result, by using the band edge of a band designed by means of the photonic crystal, absorption at a wavelength where absorption is small out of wavelengths of light absorbed by the photovoltaic layer 203 is enhanced, so that the whole photovoltaic amount is increased.